It is often necessary to know the value of a physical parameter over long distances or great areas. Pipelines are often heated to keep the contents from solidifying, so knowledge of the pipe temperature at a large number of points is quite useful. Telephone cables are often pressurized to keep moisture out, so knowledge of the pressure at a number of locations within the cable is of great value. Land is often fenced for security measures and knowledge of the fence motion at a large number of points can indicate the presence of intruders. Proper ground moisture is important for the growth of agricultural crops and knowledge of the ground moisture level at numerous locatons is important for intelligent control of irrigation equipment. Of course there are many other examples of extended or spatially distributed structures or equipment where knowledge of physical parameters measured at a great many location is useful.
Remote sensors typically utilize one or more wire pairs for connection to a central control room. The result is a great many wire pairs with attendant harnessing, connectors, conduit, expense, etc. Recent electronic advances have sought to alleviate excessive wiring through the use of multiplexing whereby one wire pair or co-axial cable connects numerous remote sensors. Any desired remote module can then be accessed by sending out an electronic address and requesting data or sending information. The difficulties with multiplexing are size and cost of the addressible modules. At present, each such module requires a separate integrated circuit to control the input and output of data. This integrated circuit is individually programmed to recognize its unique address, either through thumbwheel switches or wiring. This adds expense in the labor to adjust the switches or do the wiring and keep track of the address number of each module so it may be identified after it is packaged. The addressible integrated circuit adds cost to the module as well as size because it is required in addition to whatever other transducers and data processing circuits are needed. This precludes multiplexing in applications where a great many locations need to be monitored by relatively small, inexpensive, remote modules.
An example of a two wire multiple address, multiplex structure, entitled "Two-wire, Multiple-Transducer Communications Systems", is disclosed in U.S. Pat. No. 4,083,946. In this disclosure, each data gathering device includes a decoder operative in response to a predetermined interrogating code upon the detection of which the associated data device is actuated for transmission of data to a central receiving unit.
An example of a pre-multiplexing communication system designed to communicate with a large number of remote sensors through a relatively small number of wires is illustrated by Brenner et al U.S. Pat. No. 3,824,545. In Brenner et al, a large number of remote modules or terminal stations are connected together by a single transmission line. Each terminal station is connected to master electronics by a number of information collecting lines. Each terminal station is activated in sequence by a pulse placed on the interconnecting transmission line. Once activated, data is placed on the information collecting lines. After a time delay, determined by the choice of components within the terminal station, the station becomes deactivated. A pulse is generated, thereupon, which propagates to the subsequent stations.
Brenner et al is directed to an intrinsic timing means wherein each module includes timing circuitry and components. These components add expense, are bulky and most importantly, are sensitive to temperature and other influences causing the activation time of successive modules to vary in an unpredictable way.
A slightly more advanced approach is illustrated in Leuhrmann et al, U.S. Pat. No. 3,461,445 wherein the remote module and the master electronics both include parts of the timing mechanism. In Luehrmann et al the master electronics sends out a triggering pulse to a remote module to close its relay, then extinguishes the current holding the relay closed, and then triggers the next module through the extinguished relay before the latter can drop open. Loss of power causes the electromechanical or hard-contact switch to reopen but only after the trigger pulse has been sent to the subsequent module. In this way Luehrmann et al comprises a two part timing mechanism having a portion which is external and a portion which is intrinsic within the module.
Electromechanical or hard-contact relays such as those utilized in Luehrmann for switching functions are not long lived. The relays are subject to vibrations and are relatively large and costly. Further, each successive module in Luehrmann is triggered just after the power is re-applied to the cable but before the energized relay reopens making for a very intricate sequence of operations which has the potential for reliability problems. Thus, which Luehrmann is directed and has as an object of its invention, a simple remote sensing unit, it can be seen that this remote sensing unit intrinsically includes a portion of the timing mechanism.
Other communication systems have been developed which use relatively complex modules such as Heimbigner, U.S. Pat. No. 3,601,806. Additionally others have used communication systems wherein a sequential application of voltage is applied to activate remote modules sequentially. An example of such a system is Perry, U.S. Pat. No. 3,403,379. However disadvantages of such a system include the size of the module and the limited life of the cable because of its susceptibility to vibration. Further, since the sequential application of voltage requires that each module be activated, the number of modules is limited as a result of the voltage drop at each junction.
The instant invention utilizes a small number of conductors (wires) to connect a large number of relatively small, inexpensive, remote monitoring modules. Neither multiplex address circuitry nor individual wire pairs nor electromechanical components are necessary for each module to monitor physical parameters along the cable of the instant invention.